1. Field of the Invention
The field of the present invention relates to systems and methods for depositing chemicals onto workpieces, and the products therefrom. More particularly, the present invention relates to systems and methods for depositing silica soot on a start rod for fabricating optical fiber preforms, fused silica rods, and other optical components.
2. Background
Today's communications grade optical fiber of fused silica, SiO2, is manufactured according to three basic steps: 1) core preform or “start rod” fabrication, 2) core-with-cladding preform fabrication, and 3) fiber drawing. The core and cladding of a preform correspond in ratios and geometry to those of the ultimate glass fiber that is drawn from the preform. The composition of the core and cladding must be such that there is a lower index of refraction in the cladding than in the core. The relatively higher index of refraction of the core to a relatively lower index of refraction of the cladding is predetermined so that when the preform is drawn into a fiber, the fiber conducts light, either in single mode or in multi-mode form.
The first step is to build up a start rod, forming it into a glass that will eventually become the core of the fiber, and in some cases, part of the cladding layer. The start rod is a glass rod made of silica, SiO2, with the portion of the start rod that comprises the core being doped with a small amount of a dopant, typically Germania, GeO2. The presence of the dopant in the core increases the refractive index of the glass material compared to the surrounding outer (cladding) layer. In the second step, a cladding layer is built up on the start rod. The result of this step is a preform having a core and a cladding, which is typically about 80 mm in diameter and about one meter long. The third step is fiber drawing, where the preform is heated and stretched, and typically yields about 400 km of optical fiber.
The primary raw ingredient to fabricating the glass preform is silicon tetrachloride, SiCl4, which generally comes in a liquid form. As noted above, however, SiO2, typically in the form of glass soot, is deposited on the start rod. The chemical reactions involved in the formation of the glass soot are complex, involving, SiCl4, oxygen, O2, and the fuel gas combustion products. In all of the techniques, the silica, SiO2, comprises the cladding of the preform according, generally, to the reaction:SiCl4+O2=SiO2+2Cl2.
Generally, there are four distinct technologies for fabricating core preforms. These technologies include Modified Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD), and Plasma Chemical Vapor Deposition (PCVD). The resulting product for all of these technologies is generally the same: a “start rod” that is generally on the order of one meter long and 20 mm in diameter. The core is generally about 5 mm in diameter.
Similarly, there are generally four technologies for performing the step of adding the cladding. These technologies include tube sleeving (conceptually paralleling MCVD), OVD soot overcladding (conceptually paralleling OVD), VAD soot overcladding (conceptually paralleling VAD), and plasma (conceptually paralleling PCVD). In this step, additional cladding layers of pure or substantially pure fused silica are added to the start rod to make a final preform that can be prepared for fiber drawing.
In MCVD, the step of manufacturing the start rod is performed inside of a tube. Similarly, when the cladding step is performed, a larger tube is sleeved onto and fused to the start rod. Presently, the company, Heraeus, manufactures the tubes used for producing start rods and for sleeving onto and fusing with the start rods to make the preforms.
In OVD, when fabricating start rods, glass is deposited onto a rotating mandrel in a “soot” deposition process. The start rod is slowly built up by first depositing the germanium doped core, and then the pure silica layers. When the core deposition is completed, the mandrel is removed and then sintered into a start rod of glass.
In the process of OVD soot overcladding, where a cladding is deposited onto a fabricated start rod, the start rod is rotating and traversing on a lathe such that many thin layers of soot are deposited on the rod over a period of time. Although the SiO2 is not deposited onto the start rod as a vapor, but rather as SiO2 particles, the process is known in the art as a “chemical vapor deposition” process because the SiCl4, which reacts in the stream between the burner and the start rod to form SiO2, is input to the burner as a vapor. The porous preform that results from the OVD soot overcladding process is then sintered in a helium atmosphere at about 1500° C., into a solid, bubble-free glass blank. U.S. Pat. No. 4,599,098, issued to Sarkar, which is incorporated by reference as though fully set forth herein, provides further background on systems and techniques for OVD and OVD soot overcladding.
VAD is a process of depositing silica soot onto the end of a mandrel in a deposition station. Unlike the OVD process the mandrel is not removed prior to sintering. Furthermore, like the OVD soot overcladding technique, VAD soot overcladding is also used to deposit silica soot on a start rod to fabricate a preform. However, unlike OVD and OVD soot overcladding, VAD soot overcladding orients the mandrel and start rod vertically rather than horizontally and deposits the silica in one thick layer in one pass.
PCVD uses plasma radiation as a source of heat, and therefore is unlike the above-described processes, which use hydrogen or methane as the source of heat for the chemical reaction.
For the above-described technologies, typically any one of the core fabrication technologies may be combined with any one of the cladding fabrication technologies to generate a preform that may be used for drawing fiber.
In the OVD soot overcladding processes, one of the key measures of economic viability in comparison to the other available techniques is the deposition rate of the SiO2 on the workpiece. For example, some companies involved in optical fiber manufacturing opt for the most cost-effective method of performing the step of overcladding the start rod in the fiber manufacturing process. With respect to this step in the process, the choice is either to purchase the cladding tubes or to perform a deposition process to add the cladding.
In comparing the relative costs of the two approaches, the economics often come down to whether a particular vapor deposition system that a company is considering performs at a certain minimum deposition rate. The deposition rate may be characterized, for example, by the average grams/minute of silica soot that can be deposited on the start rod until completion (i.e., an optical fiber preform ready for sintering). Thus, above a certain threshold deposition rate, performing the soot overcladding process is likely to be economically more attractive to the company than purchasing cladding tubes. Thus, companies that manufacture systems for performing soot overcladding focus on achieving the highest possible deposition rates without compromising the quality of the preform that is produced for fiber drawing.
The factors that determine a deposition system's deposition rate are the chemical vapor delivery rate and the efficiency of chemical vapor deposition onto the workpiece. With respect to vapor delivery, key issues generally revolve around continuously and efficiently maintaining a high (e.g., greater than 200 grams/minute) delivery rate over a prolonged period (e.g., greater than 2 hours). Several methods have been described in the prior art for supplying a hydrolyzing burner with a substantially constant flow of vaporized source material entrained in a carrier gas. For example, in U.S. Pat. No. 4,314,837 issued to Blankenship (“the Blankenship reference”), a system is described that includes several enclosed reservoirs each containing liquid for the reaction product constituent. The liquids are heated to a temperature sufficient to maintain a predetermined vapor pressure within each reservoir. Metering devices are coupled to each reservoir for delivering vapors of the liquids at a controlled flow rate. The respective vapors from each reservoir are then combined before they are delivered to the burner.
This device, however, is inefficient for maintaining a substantial and steady delivery rate of chemical vapor to the burner for a prolonged chemical vapor deposition process. For a substantial and steady delivery rate, the chemical reservoir described in the Blankenship reference must be vast, and significant energy expenditure is required to maintain the chemical in the reservoir in a vapor state. On the other hand, if the chemical reservoir described in the Blankenship reference is small enough to be energy-efficient, then the deposition flow must be periodically interrupted to refill the reservoir with the chemical liquid and heat the chemical until it is in a vapor state. Because maintaining a constant, high delivery flow rate, as noted above, is a critical factor in the effective deposition rate, a need exists for a system and method of chemical vapor delivery that is energy-efficient and provides high, constant and continuous delivery of chemical vapor.
With respect to enhancing the deposition efficiency of SiO2 on the workpiece to improve the effective deposition rate, studies have been performed to characterize the flow of chemical vapor in the reaction chamber from the burners to the surface of the workpiece. One reference directed to this issue is Li, Tingye, Fiber Fabrication, pp. 75-77, Optical Fiber Communications, (Academic Press, Inc. 1985). As discussed in the above reference, because of the small size of the formed glass particles, momentum does not cause an impaction of the particles onto the surface of the workpiece. The small sizes of the glass particles would tend to force them to follow the gas stream around instead of at the preform surface. Rather, thermophoresis is the dominant mechanism for collection on the surface of the preform. As the hot gas stream and glass particles travel around the workpiece, a thermal gradient is established near the surface of the preform. Preferably, the thermal gradient is steep, effectively pulling the glass particles by a thermophoretic force towards the preform.
Various methods have been proposed to increase deposition efficiency based on establishing and maintaining the thermophoretic force, including varying the distance between the burner and the workpiece. See H. C. Tsai, R. Greif and S. Joh, “A Study of Thermophoretic Transport In a Reacting Flow With Application To External Chemical Vapor Deposition Processes,” Int. J. Heat Mass Transfer, v. 38, pp. 1901-1910 (1995). However, even applying these methods, demand for even higher deposition rates has gone unmet. A need exists therefore, for systems and methods that offer further improvement to deposition efficiency, chemical delivery and, thereby, the overall deposition rate of chemical vapor.